Re: [webcomponents] "Creating an element" needs to be more precise, for upgrades (#424)

I think we need to define a new spec hook in HTML/DOM when a new element is created, and add a new step to enqueue it for upgrades when the element doesn't match any builtins and has a valid custom element (right set of characters, contains `-`, etc...). 

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https://github.com/w3c/webcomponents/issues/424#issuecomment-193540677